This invention relates, according to an aspect thereof, to a fuel system for supplying pressurised fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine. The fuel system comprises a fuel pump, which has a pumping mechanism arranged partly in a housing containing lube oil, and a drain line connected to said housing and suitable for draining at least fuel vapour from an interior of said housing. An aspect of the invention also relates to a method for reducing fuel leakage from a corresponding fuel system.
The fuel system is particularly suitable for supplying low viscosity fuel, such as DME, to a diesel engine of a heavy truck.
In particular low viscosity fuel, such as DME, may leak past the high pressure seal of the pumping element into the camshaft chamber of the fuel pump, from where it may uncontrollably leak further out from the fuel system. One prior art solution to this problem is known from JP10281029A, where a leak gas pipe is provided between the camshaft chamber of the fuel pump and the air intake pipe of the combustion engine, so that during engine operation the gaseous fuel leakage may be removed from the camshaft chamber and safely burned off by the engine. Fuel vapour leakage to the ambient may however occur anyway to a limited extent in certain operating conditions.
There is thus a need for an improved fuel system removing the above mentioned disadvantage.
An aspect of the invention concerns fuel system for supplying pressurised fuel, in particular dimethyl ether (DME) or a blend thereof to an internal combustion engine, said fuel system comprising a fuel pump, which has a pumping mechanism arranged partly in a housing containing lube oil, and a drain line connected to said housing and suitable for draining at least fuel vapour from an interior of said housing.
An aspect of the invention is characterized in that said fuel system further comprises a lube oil supply line connected to said housing, a lube oil supply valve installed in said lube oil supply line, a seal installed between said pumping mechanism and said housing for preventing at least lube oil leakage to the outside of said housing, and a drain valve installed in said drain line, wherein both said drain valve and lube oil supply valve are configured to be closed during an engine non-running state for preventing fuel vapour leakage from said housing.
As described in connection with the prior art, fuel vapour leakage may occur to a limited extent in certain operating conditions despite the leak gas pipe, in particular in an engine non-running state when no negative pressure is generated by the power cylinders of the combustion engine. This problem is solved by an aspect of the invention by installing a drain valve in the drain line, and by closing the drain line upon entering an engine non-running state. Moreover, as a consequence of closing the drain line, relatively high pressure fuel vapour remains in said chamber of the fuel pump, and the inventive lube oil supply valve serves to prevent the confined pressurised fuel vapour from forcing the lube oil backwards through the lube oil supply line, in which event fuel vapour could propagate inside the engine via the lube oil system, or out from the engine via the crankcase ventilation, etc.
An aspect of the invention further concerns a method for reducing fuel leakage from a fuel system that is arranged to supply pressurised fuel, in particular dimethyl ether (DME) or a blend thereof, to an internal combustion engine, said fuel system comprising a fuel pump, which has a pumping mechanism arranged partly in a housing containing lube oil, and a drain line connected to said housing and suitable for draining at least fuel vapour from an interior of said housing.
The inventive method comprises the steps of connecting a lube oil supply line to said housing, installing a lube oil supply valve in said lube oil supply line, installing a seal between said pumping mechanism and said housing for preventing at least lube oil leakage to the outside of said housing, installing a drain valve in said drain line, and configuring both said drain valve and lube oil supply valve to be closed during an engine non-running state for preventing fuel vapour leakage from said housing.
According to an aspect of the invention, a lube oil fill level within said housing may at least during said engine non-running state be set such that said seal is completely immersed in lube oil during said engine non-running state for further enhancing fuel vapour sealing of said housing. The pumping mechanism of an aspect of the invention comprises at least a reciprocating pumping element and a drive mechanism for driving the pumping element by means of an external power device, such as the combustion engine. The seal installed between said pumping mechanism and said housing is arranged to prevent lube oil leakage to the outside of said housing. In the engine non-running state, confined pressurised fuel vapour is normally prevented from leaking past the seal by providing a seal, which is made of a material compatible with the fuel used, such that said seal exhibits low permeability to said fuel. However, sealing material compatible with in particular low viscosity fuels such as DME are expensive and may have lower durability than conventional seals, which are normally used for this sealing application. The solution of providing a completely immersed seal at least in the engine non-running state allows the use of conventional, and thus more economical sealing materials for the seal. Furthermore, the sealing performance of the fuel pump chamber is improved because the seal can be arranged for use exclusively as liquid oil seal, having inherently lower sensitivity to wear and deterioration as compared to a gaseous fuel seal.
According to an aspect of the invention, the lube oil supply valve may be adapted to be actuated hydraulically by the engine lube oil pressure, such that said lube oil supply valve is opened in an engine running state when the oil pressure is higher, and closed in said engine non-running state when the oil pressure is low. By configuring the lube oil supply valve to be actuated hydraulically by the engine lube oil pressure, an automatic opening control of the lube oil supply valve is realised that does not require any electronic control, thereby leading to reduced cost of the fuel system.
According to an aspect of the invention, the drain valve may be a spring loaded hydraulically operated control valve, wherein a pilot line of said control valve is connected to said lube oil supply line, such that a high oil pressure within said lube oil supply line caused by a running engine is arranged to open said drain valve, and a low oil pressure within said lube oil supply line caused by a non-running engine in combination with said spring loading is arranged to close said drain valve. By configuring the drain valve to be actuated hydraulically by the engine lube oil pressure, an automatic opening control of the lube oil supply valve is realised that does not require any electronic control, thereby leading to reduced cost of the fuel system.
According to an aspect of the invention, said seal may be made of a conventional oil seal robber material, such as nitrile rubber, hydrogenated nitrile rubber, silicone rubber, fluorinated rubber, or acrylic rubber, and said seal may be free from any DME resistant coating. Seals made of conventional, essentially non-DME resistant sealing materials are abundantly available, have low cost, and provide high and reliable sealing performance over a broad temperature range.
According to an aspect of the invention, at least a section or part of said drain line may function not only as fuel vapour drain line, but also as lube oil return line, and a lube oil separator may be provided along said drain line for separating lube oil from fuel vapour, wherein said lube oil separator is arranged downstream of said drain valve. By utilising the drain line both for draining fuel vapour and as lube oil return line, a single pipe connection to the housing is required for both said fluids, and a single control valve may be utilised for sealing the pumping mechanism chamber in the engine non-running state.
According to an aspect of the invention, said fuel system may further comprise a lube oil return valve capable of opening and closing a lube oil return line connected to said housing, wherein said lube oil return valve is adapted to be closed in said engine non-running state. Thus, this fuel system design does not utilise the drain line as lube oil return line, but provides a separate lube oil return line specifically for this purpose, i.e. provides a separate circulation path for the lube oil. Moreover, since the drain valve can no longer be used for closing the lube oil return line, an additional valve is provided in the return line for this purpose, namely the lube oil return valve. This design may thus possibly omit a lube oil separator installed in the drain line. Also, the location and dimension of the lube oil return line may be more specifically adapted to its purpose. Still more, the closing and opening control of the drain valve and lube oil return valve may be different from each other and optimised for their specific purposes.
According to an aspect of the invention, said lube oil return valve may be a spring loaded hydraulically operated control valve, wherein a pilot line of said lube oil return valve is connected to said lube oil supply line, such that a high oil pressure within said lube oil supply line caused by a running engine is arranged to open said lube oil return valve, and a low oil pressure within said lube oil supply line caused by a non-running engine in combination with said spring loading is arranged to close said lube oil return valve. By configuring the lube oil return valve to be actuated hydraulically by the engine lube oil pressure, an automatic opening and closing control of the lube oil return valve is realised that does not require any electronic control, thereby leading to reduced cost of the fuel system.
According to an aspect of the invention, said lube oil fill level may be controlled by the connection position of said lube oil return line to said housing, or by the connection position of said drain line to said housing. Depending on in which line the oil returns from the fuel pump to the low pressure lube oil reservoir of the engine, i.e. the drain line or the lube oil return line, the connection position of said line to the pumping mechanism housing may control the lube oil fill level of said chamber in a running engine, given that sufficient lube oil is supplied via the lube oil supply line. The lube oil fill level will then correspond substantially to the position of the outlet port of the drain/return line. More specifically, the lube oil fill level will correspond to the height from a lowest point of the chamber to the outlet port, measured in a vertical direction.
According to an aspect of the invention, said lube oil return line may be connected to a lower section of said housing, and said lube oil fill level is controlled by a, preferably electronic, lube oil level controller, which controls flow through said lube oil return valve. As an alternative to the design described above, where the position of the outlet port essentially determines the lube oil fill level, said fill level may instead be controlled by an electronic or other suitable lube oil level controller that controls opening and closing of the lube oil return valve. Here, the outlet port of the lube oil return valve is advantageously arranged sufficiently low in the pumping mechanism chamber, preferably at or near the bottom of the chamber. Otherwise, no efficient lube oil fill level, control may be realised.
According to an aspect of the invention, a safety means may be provided that can close said drain line. It may be advantageous to be able to close the drain line without being dependent on the drain valve, which may malfunction for same reason, or on correct control of the drain valve itself.
According to an aspect of the invention, said safety means also can close said lube oil supply line. It may be advantageous to be able to close the lube oil supply line without being dependent on the lube oil supply valve, which may malfunction for same reason, or on correct control of the lube oil supply valve itself.
According to an aspect of the invention, the fuel pump may also comprise a safety relief valve for preventing overpressure in said housing. The outlet of the safety relief valve may be connected either to the ambient or to the low-pressure part of the fuel system.
According to an aspect of the invention, said fuel pump may comprise at least one pumping chamber, and said pumping mechanism may comprise at least one pumping element for expanding and contracting said pumping chamber and a driving member for driving said at least one pumping element.
According to an aspect of the invention, said at least one pumping element may comprise a plunger, and said driving member may comprise a driveshaft with at least one cam for engagement with said at least one plunger.
According to an aspect of the invention, a single conventional high-pressure plunger seal may be provided between said at least one plunger and cylinder in which said plunger is reciprocally arranged, with said cylinder preferably being free from any drainage ports arranged to drain fuel leakage. By sealing the plunger merely with a single conventional high pressure plunger seal, without any complex fuel vapour drainage ports within the internal plunger cylinder, a less complex and more cost-effective fuel pump is provided.